The present invention relates to electrical circuits for detecting the presence or absence of a liquid and, more particularly, to a liquid presence detection circuit for control of a wet-niche type lamp assembly that is surrounded by and cooled by a liquid, the circuit interrupting power to the lamp when the level of the surrounding, cooling liquid falls below a predetermined safe level.
Various mechanical and electronic devices are known for detecting the presence or level of a liquid and operating a controlled device such as a pump, lamp, or alarm in response thereto. Typically, the mechanical devices have included a float that is operatively associated with a switch with the float actuating the switch in response to the level of the liquid. Electronic devices have included photo-optical detector circuits, thermistor-based detection circuits, and conductivity detectors which detect the presence or absence of the liquid based on optical, thermal, and conductivity characteristics of the liquid being sensed.
One application for electronic liquid detecting circuits is in controlling wet-niche type lamps which provide underwater illumination for swimming pools, fountains, and the like. In this type of illuminating lamp, the surrounding water or liquid absorbs the excess heat energy generated by the lamp filament to maintain the lamp and its housing in a preferred temperature range. Should the level of the surrounding, cooling water inadvertently drop below the level of the operating lamp, the heat energy generated by the filament that would normally be transferred to the surrounding water can cause the temperature of the lamp and its housing to increase and thereby cause an over-temperature condition. Repeated or extended over-temperature operation can materially reduce the operating life of the lamp filament, cause distortion and stress in various parts of the lamp and lamp housing, and cause many plastic parts, including plastic sealing gaskets, structural elements, and electrical insulating parts to distort, crack, or embrittle. Should the level of the cooling water be restored while the lamp is in such an over-temperature condition, the resultant thermal shock can cause further material and structural deterioration, stress, and damage to the lamp and its housing.
Prior liquid presence circuits used in wet-niche type lamps have used electronic liquid level detection circuits that interrupt the flow of electricity to the lamp to prevent low-liquid operation and consequent over-temperature condition. These circuits have typically included a selectively gatable thyristor in series with the lamp filament with the thyristor gated by a circuit that includes a capacitive impedence. While circuitry of this type is reasonably reliable and efficient in operation, the capacitive impedence in the gate circuit causes a firing angle delay in providing electrical energy to the lamp, and capacitors, as a component, are subject to variation in performance with age as well as having a higher failure rate than other electronic components.